Near Infrared Photocatalysis,Upconversion Luminescence Of Sm²⁺ And Eu³⁺ Based On Cooperative Transition

The three pillars of modern society are the energy,information,and materials,the materials provide a reliable foundation for the development of energy and information technology.With the development of science and technology,the requirement of optical functional materials is improved.In the initial cognition,the luminescent material was governed by Stokes' s law.According to law,the light with high frequency and short wavelength can stimulate the material to produce light with long wavelength and low frequency,which called Stokes luminescence.With the deepening of the research on the luminescent properties of materials,the new luminescent materials of anti-Stokes luminescence law came into being.In mid 60 s,the concept of optical frequency upconversion was put forward for the first time by Auzel [1].Since then,various types of upconversion luminescence?UCL?materials represented by inorganic luminescent materials have emerged.UCL materials have shown a good application prospect in the fields of UC lasers,infrared detection,true 3D display and biological fluorescent labels.According to the difference of the mechanism of excitation,the UC process can be divided into three main processes: excited state absorption,two-photon absorption and energy transfer,among them,the energy transfer can be divided into several processes,such as cooperative absorption,cooperative sensitization,cross relaxation and photon avalanche[2-5].As a kind of special optical frequency upconversion process,cooperative transition can make certain ions do not have energy level in the spectrum to absorb or emit photons.For the CL of dimers,D.L.Dexter has firstly given the calculation method of probability with first-order perturbation theory.Our results indicated the existence of CL of 3-Yb³⁺ ions and cooperative sensitization of 4-Yb³⁺ ions,which broadened the study range of CL and cooperative energy transfer processes [6].In this thesis,we focus on the study of the cooperative emission of Yb³⁺(Yb³⁺-dimer)and the interaction between light and matters in the energy transfer process.The main contents are as follows:1 ? A near infrared photocatalytic material based on Yb³⁺-dimer cooperative transition is obtained.A new type of NIR photocatalyst composed by cooperative luminescence agent CaF₂:Yb and semiconductor BiVO₄,in which cooperative luminescence of CaF₂:Yb in the green region was used as donor and BiVO₄ with strong absorption was acted as acceptor.The degradation of methylene blue?MB?and Rhodamine B compounds by CaF₂:1%Yb³⁺@BiVO₄ particles upon NIR radiation,and the corresponding controlled experiments demonstrated the NIR light responsive photocatalytic capability.It is noteworthy that nearly 80% degradation ratio of MB and 50% degradation of Rhodamine B were achieved with 7 h of NIR light irradiation,indicating an relatively high photocatalytic activity.Recycling experiments of photocatalysis indicate that our present CaF₂:Yb@BiVO₄ composite has good photochemical stability in spite of a long time of NIR irradiation.The energy transfer efficiency of cooperative transition is much higher than that of the re-absorption process and it can provide a new energy transfer mechanism for near infrared catalysis.2?Upconversion luminescence of Sm²⁺ ions was firstly realized.The hybrid phosphor Ba FCl_0.5Br_0.5:1%Sm²⁺-CaF₂:1%Yb³⁺ was prepared through a stepwise high temperature reaction to achieve the material containing both Yb³⁺ and Sm²⁺ ions.Upon the excitation of 980 nm diode laser,the Sm²⁺ ions emitted red upconversion fluorescence peaked at 631 nm,644 nm,665 nm,689 nm,704 nm,and 729 nm from the 5D0,1 ? 7F0,1,2 transitions,respectively.By transient dynamic analysis,we attributed the excitation of Sm²⁺ ions to the cooperation energy transfer process: two excited Yb³⁺ ions simultaneously transfer their energy to one Sm²⁺ ion.According to the fitting results,5Di?i=0,1??7Fj?j=0,1,2?transitions of Sm²⁺ ion were two-photon processes.It is really possible to achieve the UCL of changeable valence lanthanides through cooperative energy transfer of two Yb³⁺ ions(Yb³⁺-dimer),moreover,broaden the scope of the study of rare earth upconversion luminescence of rare earth ions on the upconversion luminescence,laid the foundation for further research.3?Pure red UCL material CaF₂:0.1%Yb³⁺/0.1%Eu³⁺ was obtained under near infrared excitation.In the UC spectra,the intensive emission at ⁶14 nm?5D0 ? 7F2?in the red region was observed.The color coordinate of CaF₂:0.1%Yb³⁺/0.1%Eu³⁺ was calculated with the Commission Internationale de I'Eclairage?CIE?chromaticity coordinate of?x=0.572,y=0.265?,which located in red zone.The cooperation sensitization process was so effective in the Yb³⁺/Eu³⁺ co-doped system that all the cooperative luminescence?CL?from Yb³⁺-dimers were quenched and only the red UL of Eu³⁺ ions could be seen.According to the fitting results,5D3 ? 7Fj?j=1,2,3?and 5D2 ? 7Fj?j=0,1,3?transitions were three-photon process,while 5D1 ? 7F1 and 5D0 ? 7F j?j=1,2?transitions were two-photon processe.